## 1.4 Overview

This section presents a lightning tour of Dart. The goal is to familiarize you with all the core elements of Dart without getting bogged down in detail.

Programming language constructs are often defined in a mutually recursive fashion. It is difficult to present an orderly, sequential explanation of them, because the reader needs to know all of the pieces at once! To avoid this trap, one must first get an approximate idea of the language constructs, and then revisit them in depth. This section provides that approximation.

After reading this section, you should be able to grasp the essence of the many examples that appear in later sections of the book, without having read the entire book beforehand.

Here then, is a simple expression in Dart:

3

It evaluates, unsurprisingly, to the integer 3. And here are some slightly more involved expressions:

3 + 4 (3+4)*6 1 + 2 * 2 1234567890987654321 * 1234567890987654321

These evaluate to 7, 42, 5 and 1524157877457704723228166437789971041 respectively. The usual rules of precedence you learned in first grade apply. The last of these examples is perhaps of some interest. Integers in Dart behave like mathematical integers. They are not limited to some maximal value representable in 32 or 64 bits for example. The only limit on their size is the available memory.^{1}

Dart supports not just integers but floating-point numbers, strings, Booleans and so on. Many of these built-in types have convenient syntax:

3.14159 // A oating-point number `a string' "another string - both double quoted and single quoted forms are supported" 'Hello World' // You've seen that alreadytruefalse// All the Booleans you'll ever need [] // an empty list [0, 1.0,false, 'a', [2, 2.0,true, \b"]] // a list with 5 elements, the last of which is a list

As the above examples show, single-line comments are supported in Dart in the standard way; everything after `//` is ignored, up to the end of the line. The last two lines above show literal lists. The first list is empty; the second has length 5, and its last element is another literal list of length 4.

Lists can be indexed using the operator `[]`

[1, 2, 3] [1]

The above evaluates to 2; the first element of a list is at index 0, the second at index 1 and so on. Lists have properties `length` and `isEmpty` (and many more we won’t discuss right now).

[1, 2, 3]. length // 3 [].length // 0 [].isEmpty // true ['a'].isEmpty // false

One can of course define functions in Dart. We saw our first Dart function, the `main()` function of “Hello World”, earlier. Here it is again

main(){ print(`Hello World'); }

Execution of a Dart program always begins with a call to a function called `main()`. A function consists of a header that gives its name and any parameters (our example has none) followed by a body. The body of `main()` consists of a single statement, which is a call to another function, `print()` which takes a single argument. The argument in this case is the string literal `‘Hello World’`. The effect is to print the words “Hello World”.

Here is another function:

twice(x) => x * 2;

Here we declare `twice` with a parameter `x`. The function returns `x` multiplied by 2. We can invoke `twice` by writing

twice(2)

which evaluates to 4 as expected. The function `twice` consists of a signature that gives its name and its formal parameter `x`, followed by => followed by the function body, which is a single expression. Another, more traditional way to write `twice` is

twice(x) {returnx * 2; }

The two samples are completely equivalent, but in the second example, the body may consist of zero or more statements—in this case, a single **return** statement that causes the function to compute the value of `x*2` and return it to the caller.

As another example, consider

max(x, y){if(x > y)returnx;else returny; }

which returns the larger of its two arguments. We could write this more concisely as

max(x, y) => (x > y) ? x : y;

The first form uses an **if** statement, found in almost every programming language in similar form. The second form uses a conditional expression, common throughout the C family of languages. Using an expression allows us to use the short form of function declarations.

A more ambitious function is

maxElement(a) {varcurrentMax = a.isEmpty ?throw'Maximal element undefined for empty array' : a[0];for(var i = 0; i < a.length; i++) { currentMax = max(a[i], currentMax); }returncurrentMax; }

The function `maxElement` takes a list `a` and returns its largest element. Here we really need the long form of function declaration, because the computation will involve a number of steps that must be sequenced as a series of statements. This short function will illustrate a number of features of Dart.

The first line of the function body declares a variable named `currentMax`, and initializes it. Every variable in a Dart program must be explicitly declared. The variable `currentMax` represents our current estimate of the maximal element of the array.

In many languages, one might choose to initialize `currentMax` to a known value representing the smallest possible integer, typically denoted by a name like `MIN INT`. Mathematically, the idea of “smallest possible integer” is absurd. However, in languages where integers are limited to a fixed size representation defined by the language, it makes sense. As noted above, Dart integers are not bounded in size, so instead we initialize `currentMax` to the first element of the list. If the list is empty, we can’t do that, but then the argument `a` is invalid; the maximal element of an empty list is undefined. Consequently, we test to see if `a` is empty. If it is, we raise an exception, otherwise we choose the first element of the list as an initial candidate.

Exceptions are raised using a **throw** expression. The keyword **throw** is followed by another expression that defines the value to be thrown. In Dart, any kind of value can be thrown—it need not be a member of a special `Exception` type. In this case, we throw a string that describes the problem.

The next line begins a **for** statement that iterates through the list.^{2} Every element is compared to `currentMax` in turn, by calling the `max` function defined earlier. If the current element is larger than `currentMax`, we set `currentMax` to the newly discovered maximal value.

After the loop is done, we are assured that `currentMax` is the largest element in the list and we return it.

Until now, this tutorial has carefully avoided any mention of terms like object, class or method. Dart allows you to define functions (such as `twice`, `max` and `maxElement`) and variables outside of any class. However, Dart is a thoroughly object-oriented language. All the values we’ve looked at — numbers, strings, Booleans, lists and even functions themselves are objects in Dart. Each such object is an instance of some class. Operations like `length`, `isEmpty` and even the indexing operator `[]` are all methods on objects.

It is high time we learned how to write a class ourselves. Behold the class `Point`, representing points in the cartesian plane:

classPoint {varx, y; Point(a, b){x = a; y = b;} }

The above is an extremely bare-bones version of `Point` which we will enrich shortly. A `Point` has two instance variables (or fields) `x` and `y`. We can create instances of `Point` by invoking its constructor via a **new** expression:

varorigin =newPoint(0, 0);varaPoint =newPoint(3, 4);varanotherPoint =newPoint(3, 4);

Each of the three lines above allocates a fresh instance of `Point`, distinct from any other. In particular, `aPoint` and `anotherPoint` are different objects. An object has an identity, and that is what distinguishes it from any other object.

Each instance of `Point` has its own copies of the variables `x` and `y`, which can be accessed using the dot notation

origin.x // 0 origin.y // 0 aPoint.x // 3 aPoint.y // 4

The variables `x` and `y` are set by the constructor based on the actual parameters provided via **new**. The pattern of defining a constructor with formal parameters that correspond exactly to the fields of an object, and then setting those fields in the constructor, is very common, so Dart provides a special syntactic sugar for this case:

classPoint {varx, y; Point(this.x,this.y); }

The new version of `Point` is completely equivalent to the original, but more concise. Let’s add some behavior to `Point`

classPoint {varx, y; Point(this.x,this.y); scale(factor) =>newPoint(x * factor, y * factor); }

This version has a method `scale` that takes a scaling factor `factor` as an argument and returns a new point, whose coordinates are based on the receiving point’s, but scaled by `factor`.

aPoint.scale(2).x // 6 anotherPoint.scale(10).y // 40

Another interesting operation on points is addition

classPoint {varx, y; Point(this.x,this.y); scale(factor) =>newPoint(x * factor, y * factor);operator+(p) =>newPoint(x + p.x, y + p.y); }

Now we can write expressions like

(aPoint + anotherPoint).y // 8

The operator `+` on points behaves just like an instance method; in fact, it is just an instance method with a strange name and a strange invocation syntax.

Dart also supports static members. We can add a static method inside of `Point` to compute the distance between two points:

staticdistance(p1, p2) { var dx = p1.x - p2.x; var dy = p1.y - p2.y;returnsqrt(dx * dx + dy * dy); }

The modifier **static** means this method is not specific to any instance. It has no access to the instance variables `x` and `y`, as those are different for each instance of `Point`. The method makes use of a library function, `sqrt()` that computes square roots. You might well ask, where does `sqrt()` come from? To understand that, we need to explain Dart’s concept of modularity.

Dart code is organized into modular units called libraries. Each library defines its own namespace. The namespace includes the names of entities declared in the library. Additional names may be imported from other libraries. Declarations that are available to all Dart programs are defined in the Dart core library which is implicitly imported into all other Dart libraries. However, `sqrt()` is not one of them. It is defined in a library called `dart:math`, and if you want to use it, you must import it explicitly.

Here is a complete example of a library with an import, incorporating class `Point`

librarypoints;import'dart:math';classPoint {varx, y; Point(this.x,this.y); scale(factor) =>newPoint(x * factor, y * factor);operator+(p) =>newPoint(x + p.x, y + p.y);staticdistance(p1, p2) { var dx = p1.x - p2.x; var dy = p1.y - p2.y;returnsqrt(dx * dx + dy * dy); } }

We have declared a library called `points` and imported the library `dart:math`. It is this import that makes `sqrt` available inside the `points` library. Now, any other library that wants to use our `Point` class can import `points`.

A key detail to note is that the **import** clause refers to a string `‘dart:math’`. In general, imports refer to uniform resource indicators (URIs) given via strings. The URIs point the compiler at a location where the desired library may be found. The built-in libraries of Dart are always available via URIs of the form `‘dart:`˙`’`, where ˙ denotes a specific library.

There is a lot more to Dart than what we’ve shown so far, but you should have a general idea of what Dart code looks like and roughly what it means. This background will serve you well as we go into details later in the book.